The photochromic mesoporous silica/ molybdphosphoric acid composite thin films were prepared by evaporation induced self-assembly process and wetness impregnation. The results indicate that the PMo molecules are homogeneously distributed and immobilised inside silica mesopores modified by organic amine. The composite thin films change from colorless to blue under ultraviolet irradiation. It is shown that the charge transfer occurs by reduction of heteropolyanions accompanying the formation of heteropolyblues, and the bleaching process of composite thin films is closely related to the presence of oxygen.